| US 7,455,736 B2 | ||
| High tensile strength steel sheet excellent in processibility and process for manufacturing the same | ||
| Takahiro Kashima, Kakogawa (Japan); and Koichi Sugimoto, Nagano (Japan) | ||
| Assigned to Kabushiki Kaisha Kobe Seiko Sho, Kobe-shi (Japan); and Shinshu University, Matsumoto-shi (Japan) | ||
| Filed on Aug. 30, 2004, as Appl. No. 10/928,176. | ||
| Claims priority of application No. 2003-307463 (JP), filed on Aug. 29, 2003; and application No. 2003-351006 (JP), filed on Oct. 09, 2003. | ||
| Prior Publication US 2005/0081966 A1, Apr. 21, 2005 | ||
| Int. Cl. C22C 38/06 (2006.01); C22C 38/04 (2006.01); C21D 8/02 (2006.01) | ||
| U.S. Cl. 148—320 [420/8; 420/120; 428/659; 148/533; 148/651] | 21 Claims |
| 1. A high tensile strength steel sheet comprising a matrix and a second phase, wherein the matrix comprises at least tempered
martensite or tempered bainite, and optional ferrite, and the second phase comprises retained austenite,
wherein the retained austenite comprises lath-like retained austenite having a long axis/short axis ratio of 3 or larger at
60% or larger by area relative to total retained austenite, and
wherein
(1) the steel comprises C: 0.10 to 0.6 mass %, Si: 1.0 mass % or smaller, Mn: 1.0 to 3 mass %, Al: 0.3 to 2.0 mass %, P: 0.02
mass % or smaller, and S: 0.03 mass % or smaller,
(2) a volume rate of retained austenite, obtained by a saturated magnetization measuring method, is 10 to 40% by volume, and
(3) a relationship of a carbon amount in mass % in the steel, a volume rate (fγR) of retained austenite and a carbon concentration
(CγR) of the retained austenite satisfies the following equation (I):
(fγR×CγR)/C>50 (I).
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